Tibial implants generally form part of a knee prosthesis and are typically associated with a femoral implant fixed to the lower end of the femur. A pad is on the tray of the tibial implant and articulated against the femoral implant. The pad can either be fixed or mobile relative to the tray.
The use of an offset stem tibial implant aims to optimize the medio-lateral and antero-posterior positioning of the tray relative to the tibial medullary canal. The medullary canal dictates the position of the stem of the implant. Depending on the knee being operated upon, it is often desirable that the longitudinal axis of the stem and the centre axis of the tray are parallel, but offset, relative to one another. The offset can in practice be several millimetres.
Many tibial implants currently have an offset stem attached to the distal end of a pin of the tray. For example, the distal end of an extension of lengthened material extends over the centre axis of the tray from the distal face of the tray and is introduced into the epiphyseal end of the medullary canal of the tibia. The length of these pins generally extends over 20 to 40 millimeters (mm), depending on the implants. Consequently, the interface between the stem and the pin is located further into the medullary canal than is desirable.
To offset the axis of the stem relative to that of the pin, the proximal part of the stem has an angled shape so that its proximal end is offset relative to its distal part received in the diaphyseal part of the medullary canal, as proposed in U.S. Pat. No. 6,146,424 or U.S. Pat. No. 5,290,313. Alternatively, an angled adapter element is placed fixedly between the distal end of the pin and the proximal end of the stem, as proposed in U.S. Pat. No. 6,162,255 or EP-A-0 853 930.
These two solutions have the drawback of placing the effective area of the offset of the two axes at a significant depth of the tibial medullary canal. The surgeon therefore has to prepare this canal in advance, by removing the bone tissue in an area of the tibia, which is quite fragile. Furthermore, when the offset is implemented, the distribution of the forces produced in the region of this offset area in the centre of the epiphyseal part of the medullary canal deviates from the anatomical behaviour observed in a healthy tibia.
U.S. Pat. No. 6,214,052 discloses a tibial implant with a reversible offset stem. This implant includes an adapter element that links a tibial tray and the stem that can be assembled in two possible configurations. That is, the tibial axis of the stem has to be offset to the right or to the left with respect to the tibial axis associated to the tray. For this purpose, the adapter element includes a proximal aperture in which is housed a protrusion extending from the distal face of the tray, without interposition of a pin of the here above discussed type.
In use, however, the left/right reversibility of the assembling between the tray and the adapter element is provided due to the fact that the respective cross sections of the protrusion and of the aperture are oblong, which enables no freedom for adjusting the relative positioning of the two tibial axes, except the freedom for choosing between the two opposed left/right configurations. Consequently, a lot of adapter elements with different respective configurations for the angular offset between their proximal and distal parts are necessarily provided for the surgeon, which results in problems of costs, logistics and efficiency.